Over the years, the heavy duty trucking market has adopted the diesel engine as its preferred power source due to both its excellent longevity and its economy of operation. Specialized lubricants have been developed to meet the more stringent performance requirements of HD diesel engines compared to passenger car engines.
Starting in the late 1980's, changes in the U.S. emission laws began to force significant changes in heavy duty diesel engine design. Although not all of these changes had an impact on lubricants, taken as a whole, they generally required higher quality lubricants to maintain acceptable performance in the redesigned engines.
The American Petroleum Institute (API) has responded to these increasing performance requirements by raising the heavy duty oil quality levels from CD to CE, CF-4 and, most recently, CG-4.
As we look to the future, HD diesel emissions limits will tighten once again in 1998 with a 20% reduction in NOx. ASTM is already hard at work on a new performance category identified as PC-7 (proposed category-7), aimed at meeting the performance needs of 1998 engines.
The PC-7 category is being designed to give significant improvements in diesel detergency, soot and wear control for HD lubricants. Several new diesel engine tests are being developed for this category such as the:                Caterpillar 1P single cylinder test engine to evaluate piston deposits        Mack T-9 six cylinder test to examine ring and liner wear        Cummins M11 test to evaluate soot-related valve train wear, filter plugging and sludge.        
The PC-7 category will also include some of the engine tests from previous categories but with more stringent test limits.